Can’t believe it’s deicing season already. I still have my thunderstorm avoidance season decorations up. Anyway, why do we deice, what are they using, how effective is it & is it safe? When ice forms on the wing, it changes the aerodynamic properties. How much? Nobody knows because ice can form differently every time…but that’s kind of the point. We know how much lift we will generate w/ a “clean” wing, but an iced wing is a big question mark. Will it fly? Maybe! But maybe not, and you’d rather not find out it’s the latter when you try to lift off from the runway. So, we deice the wings, tail & fuselage. If the temperature is warm enough & it’s not snowing or raining, just removing the ice is enough. Sometimes you can do that with just hot air, or hot water. Usually it’s some kind of ethylene glycol, though, with additives. I’ll cover that in a minute. If there is still precipitation & it’s cold, we need to remove the ice & make sure new ice can’t form…we’re anti-icing (for simplicity, let’s just call it deicing too). Glycol is again used for this purpose, but now we’re adding stuff like thickening agents to make it stick to the wing. Various other additives are present, as well, like orange dye…that helps the deicing truck see what they’ve sprayed & what they haven’t. There are 4 types of deicing fluid w/ varying differences. The biggest difference is how long the fluid will stick to the jet & prevent new ice from forming. This is called “Holdover Time” and every fluid has changing holdover times based on the weather conditions. Things like temperature & type of precipitation…light snow doesn’t have much impact on the holdover time, but freezing rain has a major impact. In fact, freezing rain can get to the point that your holdover time is essentially zero & there is no point to even trying to go fly. The FAA publishes new tables of holdover times every year based on tests they run. At my airline, we have an app we use…we enter the time deicing started, the type of fluid used & verify that our flight number & location is correct; the app then pulls the weather (from a very detailed source, things like pressure, wind & exact precipitation rates matter), then spits out what our holdover time is. If you can’t take off before the holdover ends, you’re out of luck & can go get deiced again, then do it all over. Unfortunately, the kind of weather that calls for this stuff is also affecting the runways & the arrival / departure rates of an airport, so you’re likely to see delays. The runways have their own treatment chemicals which helps prevent the buildup of snow / ice, but that can be overtaken by holdovers, too. As painful as it is, even just a layer of frost can drive you to deice. That’s not always the case, though; the engineers did the math & identified areas of the wing that frost isn’t a problem. These are often painted in black boxes on the top of the wing…if the frost isn’t all inside the black box, you’re good to go. In part, the cold-soaked fuel – which is stored in the wings – will generate frost when you come down from altitude, even on warm days, and we need to be able to distinguish that from environmental icing. Now, you might be wondering how we fly through icing if we don’t get deiced in the air. The answer is that once we are flying, the airflow over the wing mostly keeps ice from forming on the wing. It does want to form on the leading edges of the wing, but that shiny leading edge is not painted because the engine is taking hot air from the combustion chamber & pushing it out along that leading edge, keeping ice from forming. The front of the engines have the same…the shiny, unpainted metal bits can get hot when we flip a switch to redirect a little bit of what would have been thrust into the tubes that warm the unpainted bits up. (Other unpainted bits are heated electrically, like pitot tubes & static ports, which sample the air & tell us speed & altitude) Damn, out of space…next tweet…

NTSB issues the preliminary report for its ongoing investigation of the Nov. 4 crash of a UPS Boeing MD-11F airplane in Louisville, Kentucky. Download the report PDF: ntsb.gov/investigations/Page…